This invention is in the field of wireless telephones, and is more specifically directed to audio output circuitry for wireless telephones.
The popularity of wireless telephones has greatly increased in recent years, because of many recent improvements in the architecture and performance of wireless telephone handsets. These improvements have provided improved audio fidelity, greater reception range, and increased feature sets, while still reducing the cost of the handset and the wireless service. Improvements in battery technology and also lower power consumption of the telephone circuitry, have improved wireless telephone battery life, which is also a factor in the increased popularity of wireless telephony.
Handsfree accessories have also increased the usability of wireless telephones. As is well-known, the typical handsfree accessory is a headset, including both a microphone and an earpiece. In particular, these accessories also reduce some of the safety hazard associated with using the telephone while driving an automobile. The convenience of modern handsfree accessories is evident from the numerous wireless telephone users that carry out wireless telephone conversations using handsfree accessories even while not driving.
It has been observed, however, that a somewhat loud “pop” sound can be produced at the audio output of a wireless telephone when it is first turned on. This noise is caused by a single-ended output of an audio amplifier making an abrupt voltage transition as it becomes biased. This abrupt transition produces a relatively large output current spike with a sharp rise time, and thus with a wide frequency range including audible frequencies. As this spike propagates to the telephone speaker or earpiece, the result is a “pop” noise. This “pop” noise generally does not occur at the speaker of a conventional wireless telephone, which is a symmetric load receiving a differential output signal; during power-on of the speaker amplifier, both of the differential inputs to the speaker see the same voltage variation, so there is no current variation through the speaker load and therefore no “pop”. However, the “pop” noise can be uncomfortably loud through a headset earpiece, which is driven by a single-ended output.
“Pop” noise is also noticeable in other situations. One such situation is in the event of a brief power-down of the amplifier. Typically, one or more capacitors couple the speaker to the audio amplifier, to keep DC components from the amplifier output from propagating to the speaker. If this capacitance discharges too quickly on power-down, a “pop” effect in audible frequencies can be generated. Conversely, if this capacitance does not fully discharge on power-down, remaining charge on the capacitors upon the next power-up of the amplifier can cause a voltage surge that is sufficient to “pop” the headset speaker.
Prior attempts to reduce this “pop” noise, in the context of wireless telephones, has involved the use of passive components external to the integrated circuits including the audio amplifier. These external components of course add to the manufacturing cost of the wireless telephone, and are also not conducive to the reduction of the form factor of the handset, the reduction of which is an ongoing desire in the art. Additionally, the relatively large size of off-chip passive components dissipate significant power, which is of particular concern in battery-powered mobile devices such as wireless telephones.
By way of further background, U.S. Pat. No. 5,537,081 describes audio amplifier circuitry in which the audio amplifier has an input stage-that operates at zero common-mode input voltage, and has an output stage to provide the low voltage end of a voltage swing near ground. This reference also discloses a shunt circuit system to discharge external capacitors, preventing “pop” noise from residual charge at power-up.